1. Field of the Invention.
This invention relates to methods and apparatus for simulating well bore conditions before and after cementing therein.
2. Description of the Prior Art.
A variety of drilling fluids are used in the drilling of well bores. Generally, the fluids are solids containing water based gels or hydrocarbon based fluids which can be non-weighted or weighted with particulate weighting materials such as barite. Most drilling fluids contain gelled bentonite and/or one or more natural or synthetic polymeric additives such as polysaccharides or polysaccharide derivatives. The polymeric additives add viscosity to the fluid and are included to impart sufficient carrying capacity and thixotropy whereby the fluids can transport cuttings to the surface and prevent the cuttings from settling appreciably when circulation is interrupted. Natural and synthetic polymeric fluid loss control additives are also commonly utilized in drilling fluids, e.g., polysaccharides, polysaccharide derivatives, polyacrylic acids, polyvinyl alcohol and the like.
One of the most important functions of a drilling fluid is to seal off the walls of the well bore so that the drilling fluid is not lost into highly permeable subterranean zones penetrated by the well bore. This is accomplished by the deposit of a filter cake of solids from the drilling fluid, dehydrated drilling fluid and gelled drilling fluid over the surfaces of the well bore whereby the solids bridge over the formation pores and do not permanently plug the pores. During the drilling of a well bore, the drilling fluid is continuously circulated down the drill pipe, through the drill bit and back to the surface through the annulus between the drill pipe and the walls of the well bore. After a well bore reaches total depth, the circulation of the drilling fluid is stopped while the well is logged and a pipe string, e.g., casing or a liner is run in the well bore. During the shut down period, additional gelled and dehydrated drilling fluid and filter cake are deposited on the walls of the well bore. As a result of the polymeric viscosifiers and additives in the drilling fluid, the filter cake formed is generally very stable and difficult to remove.
After a string of pipe is run in the well bore, primary cementing operations are usually performed therein. That is, the string of pipe is sealed in the well bore by placing a cement composition in the annulus between the pipe and the walls of the well bore. The cement composition sets into a hard impermeable mass and it is intended to bond the pipe to the walls of the well bore whereby the annulus is sealed and fluid communication between subterranean zones or to the surface by way of the annulus is prevented. However, in order for the cementing of the pipe in the well bore to be successful, the filter cake comprised of gelled and dehydrated drilling fluid and solids deposited on the walls of the well bore must be removed. If appreciable filter cake remains on the walls of the well bore, the cement composition utilized will not properly bond thereto and fluid leakage through the annulus and other major problems will result.
Heretofore, flushes have been pumped through the annulus between the pipe and the walls of the well bore prior to cementing in order to remove filter cake therefrom. The flushes have been pumped through the annulus at high rates so that they are in turbulence as they contact the filter cake, and surfactants have been included in the flush fluids to lower surface tension and enhance the penetration of the flush fluids into the filter cake. While these methods have been believed to achieve success, the only measure of that success has been the subsequent detection or non-detection of annulus leaks over time.
Thus, there are needs for apparatus and methods that can quickly and easily be utilized for testing the effectiveness of flush fluids in removing filter cake from well bores as well as the strengths of the subsequent bonds obtained between cement compositions and the walls of the well bores.